User guide

User Guide 101
Wireless Setup
Log authentication events
An authentication event occurs when a wireless computer tries to connect to an Edge wireless interface. To
have the Edge record these events in the log file, select the Log Authentication Events check box.
Change the fragmentation threshold
The Firebox X Edge e-Series Wireless allows you to set the maximum frame size it can send without
fragmenting the frame. This is called the fragmentation threshold. This setting is rarely changed. It is set at the
default maximum frame size of 2346, which means that it will never fragment any frames that it sends to
wireless clients. This is best for most environments.
About the frame size
A collision happens when two devices that use the same medium transmit packets at exactly the same time.
The two packets can corrupt each other, and the result is a group of unreadable pieces of data. If a packet
results in a collision, the packet is discarded and it must be transmitted again. This adds to the overhead on
the network and can reduce the throughput or speed of the network.
Larger frames are more likely to collide with each other than smaller frames. You can make the wireless
packets smaller by lowering the fragmentation threshold on the Firebox X Edge. If you lower the maximum
frame size, it can reduce the number of retransmissions caused by collisions, and lower the overhead caused
by retransmissions. However, making frames smaller introduces a different kind of overhead.
Smaller frames introduce more overhead on the network too. This is especially true on a wireless network,
because every fragmented frame sent from one wireless device to another wireless device requires the
receiving device to acknowledge the frame. In times of high packet error rates (over five or ten percent
collision or errors), lowering the fragmentation threshold can help improve performance of the wireless
network. The time that is saved from reducing re-transmissions can be enough to offset the extra overhead
added by using smaller packets. This can result in higher throughput.
If the rate of packet error is low and you lower the fragmentation threshold, wireless network performance will
decrease. This is because lowering the threshold adds protocol overhead and reduces protocol efficiency.
If you want to experiment, start with the default maximum 2346, and lower the threshold a small amount at a
time. To get the most benefit, you must monitor the network for packet errors at different times of the day.
Compare the effect on network performance of lowering the threshold when errors are very high with the
effect on performance when errors are moderately high. In general, we recommend that you leave this setting
at its default of 2346.
Change the RTS threshold
RTS/CTS (Request To Send / Clear To Send) is a function that helps prevent problems when wireless clients can
receive signals from more than one wireless access point on the same channel. The problem is sometimes
known as hidden node.
We do not recommend that you change the default RTS threshold. When the RTS Threshold is set to the
default of 2346, it effectively turns off RTS/CTS.
If you must change the RTS threshold, adjust it incrementally. Start by lowering it a small amount each time.
After each change, allow enough time to decide whether the change in network performance is positive
before you change it again. If you lower this value too much, you can introduce more latency into the network,
as Requests to Send are increased so much that the shared medium is reserved more often than necessary.